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Scientia Silvae Sinicae ›› 2021, Vol. 57 ›› Issue (11): 94-104.doi: 10.11707/j.1001-7488.20211110

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Bivariate Joint Distribution of DBH and Age of Moso Bamboo Based on Copula Density Function

Enbin Liu1,Hongwen Yao3,Zexi Ren2,Guomo Zhou1,*,Huaqiang Du1   

  1. 1. State Key Laboratory of Subtropical Silviculture, Zhejiang A & F University Key Laboratory of Carbon Cycling in Forest Ecosystems and Carbon Sequestration of Zhejiang Province College of Environmental and Resource Sciences, Zhejiang A & F University Hangzhou 311300
    2. Department of Geography, University College London London WC1E 6BT
    3. Forest Resource Monitoring Center of Zhejiang Province Hangzhou 310020
  • Received:2020-09-29 Online:2021-11-25 Published:2022-01-12
  • Contact: Guomo Zhou

Abstract:

Objective: In view of the shortcomings of the bivariate joint distribution function commonly used in the investigation of forest structure characteristic factors, a bivariate distribution(density)function with a low applicable condition, wide adaptation range and great application value was selected to provide a reference for accurately measuring the joint distribution of forest structure characteristic factors. Method: Five commonly used bivariate Copula density functions, bivariate Sbb functions and bivariate Weibull distribution functions, were selected to describe the bivariate joint density of DBH and age of moso bamboo(Phyllostachys edulis) based on 177 continuous inventory plots in Zhejiang Province in 2009, and the measurement accuracy of each function was compared and analyzed. The optimal Copula function of DBH and age was selected based on the metrics of bivariate frequency histogram and AIC. The bivariate Copula joint density model of DBH and age for moso bamboo in Zhejiang Province was established. The goodness of fit of the model was tested using Kolmogorov test. Result: The coefficient of determination(R2) of bivariate Weibull distribution function and the bivariate Sbb function was 0.990 1 and 0.736 2, respectively, and the R2 of bivariate Gumbel Copula density function was 0.984 1 with the lowest AIC value of -19.519 6. The maximum deviations of the cumulative value of the bivariate Gumbel Copula function, the bivariate Weibull distribution function and the bivariate Sbb function were 0.007 0, 0.015 8 and 0.078 1. The critical significance value was 0.179 8. Conclusion: Bivariate Gumbel Copula probability density function is the best Copula function for representing the joint distribution of moso bamboo DBH and age. The measurement accuracy of bivariate Weibull distribution function is the highest, but the number of the parameters used is more than that of those used in the bivariate Copula function. Therefore, iterative parameter of bivariate Copula function is easier to be converged. The accuracy of bivariate Sbb function is the lowest. The joint distribution of DBH and age of moso bamboo all obey these three distribution functions. Bivariate Copula function is suitable for arbitrary edge distribution of DBH and age, that is, it does not need to determine the class of marginal distribution function. When there are only marginal distribution values of DBH and age, the joint density value of DBH and age could be obtained using the bivariate Copula function. Thus, the bivariate Copula function has a wider applicability and a higher application value than the commonly used binary distribution function.

Key words: moso bamboo, bivariate distribution, Copula density function, biomass, structure characteristics of DBH and age

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